Postfix XFORWARD Howto

Logging after SMTP-based content filter. With the
deployment of Internet->MTA1->filter->MTA2 style
content filter applications, the logging of client and message
identifying information changes when MTA1 gives the mail to
the content filter. To simplify the interpretation of MTA2
logging, it would help if MTA1 could forward remote client
and/or message identifying information through the content
filter to MTA2, so that the information could be logged as part
of mail handling transactions.

This extension is implemented as a separate EMSTP command, and
can be used to transmit client or message attributes incrementally.
It is not implemented by passing additional parameters via the MAIL
FROM command, because doing so would require extending the MAIL
FROM command length limit by another 600 or more characters beyond
the space that is already needed to support other extensions such
as AUTH.